1. Field of the Invention
The present invention relates to an optical waveguide, a holographic medium using an optical waveguide, a method and system for controlling the angle of an incident beam onto such an optical device, and a holographic storage and retrieval method and system.
2. Description of the Related Art
Recently, holographic storage has become a focus of attention because of its capability of providing multiple data storage in a single medium, which produces high-density storage.
A known concrete example of holographic multiple data storage methods is a so-called volume holographic method using multiple angles (refer to J. F. Heanue et al., “Volume Holographic Storage and Retrieval of Digital Data”, Science, vol. 265, pp. 749–752, 1994).
FIG. 73 shows an example of a conventional volume holographic storage and retrieval method. In the figure, reference numeral 400 indicates a holographic medium made of an optical storage material, reference numeral 6 indicates an object beam, reference numeral 71 indicates a reference beam, and reference numeral 7 indicates a retrieved beam. The reference beam 71 and the object beam 6 have the same wavelength which sensitizes the holographic medium 400.
In order to perform the storage process in FIG. 73, an object beam 6 which carries data to be stored in the medium 400 and a reference beam 71 which has a specific wavefront are simultaneously emitted onto the medium 400. Accordingly, an interference fringe pattern produced by the object beam 6 and the reference beam 71 is stored in the medium 400. That is, the data carried by the object beam 6 is stored as a hologram.
In the retrieval process, a reference beam 71 which has the same wavefront as that employed in the storage process is emitted onto the medium 400. Accordingly, the reference beam 71 is diffracted by the interference fringe pattern stored in the medium 400, and the diffracted beam is observed as a retrieved beam 7. In this process, the wavefront of the retrieved beam 7 indicates the wavefront of the object beam 6 which was used in the storage process; thus, the data which was carried by the object beam 6 can also be retrieved by the retrieved beam 7.
If the reference beam 71 has a plane wave and is made incident on the medium 400 multiple times while the incident angle is changed for each time, each produced interference fringe pattern stored in the medium 400 is different and independent according to each incident angle, thereby realizing multiple angle storage.
In the retrieval process, a reference beam 71, which has the same incident angle as one of the incident angles used in the storage process, is made incident onto the medium 400 so that a retrieved beam 7 which corresponds only to that incident angle is obtained. Therefore, only desired data among the data which were stored using multiple angles can be independently retrieved.
The holographic storage has a very high angular resolution; thus, data having a very slight incident-angle difference can be independently stored. This feature realizes improved multiple storage and high-density recording.
However, when data is stored using multiple angles in the system shown in FIG. 73, the incident angle of the reference beam 71, which is incident on the medium 400 from an external device, should be accurately controlled in the storage and retrieval processes; thus, an optical system (having a mirror or the like) for emitting the reference beam 71 must be driven with sufficiently high accuracy. Therefore, a large and expensive mechanism is necessary.
As another conventional example, a so-called optical waveguide holographic medium is shown in FIG. 74 (refer to T. Suhara et al., “Waveguide Holograms”, the proceedings of the IEICE (Institute of Electronics, Information and Communication Engineers), Vol. J60-C, No. 4, pp. 197–204, 1977). This optical waveguide holographic medium has a core layer 1, a storage layer (made of an optical storage material) stacked on the core layer 1, and a cladding layer 2.
As shown in FIG. 75, a reference beam 5 is input from an end face of the core layer 1, so as to transmit a transmitted beam 500 through the core layer 1. An object beam 6 is also input into the holographic medium in a direction perpendicular to the plane of the core layer 1. Accordingly, an interference fringe pattern is produced by the object beam 6 and evanescent light 520 which is produced by the reference beam 5, and the interference fringe pattern is stored as a hologram in the storage layer 4.
In the retrieval process, as shown in FIG. 75, when a reference beam 5 is emitted onto the core layer 1, evanescent light 520 is produced by the reference beam 5 and is leaked towards the outside of the core layer 1. This leaked evanescent light 520 passes through the storage layer 4 and is diffracted by the stored interference fringe pattern so that a retrieved beam 7, which corresponds to the object beam 6 in the storage process, is produced.
Another method is known in which optical waveguides are multilayered so as to have a multilayered optical waveguide holographic medium (refer to Japanese Unexamined Patent Application, First Publication No. Hei 9-101735, or the like).
FIG. 76 shows an example of such a multilayered optical waveguide holographic medium. In FIG. 76, each storage layer 4 is positioned between a pair of the core layer 1 and the cladding layer 2.
In order to store a hologram in the medium shown in FIG. 76, a reference beam 5 is input from an end face of a core layer 1 via a lens 44, and an object beam 6 is simultaneously input into the medium in a direction perpendicular to the waveguide plane of the planar optical waveguide, thereby storing a hologram in the storage layer 4 provided between the core layer 1 and the cladding layer 2.
In order to retrieve the hologram stored in the multilayered optical waveguide holographic medium shown in FIG. 76, the reference beam 5 is input from the end face of the core layer 1, so as to obtain a retrieved beam 7 based on the same principle as that applied to the structure shown in FIG. 75.
Another conventional example for obtaining a retrieval-only multiple storage holographic medium by forming multilayered optical waveguides, each having a scattering factor, is known (refer to Japanese Unexamined Patent Application, First Publication No. Hei 11-337756 or the like).
FIG. 77 shows an example of such a retrieval-only multiple storage holographic medium having multilayered optical waveguides which have scattering factors. In FIG. 77, reference numeral 325 indicates a scattering factor formed in a boundary surface of one of the core layer 1 and the cladding layer 2, where the scattering factor is provided for retrieving a hologram. More specifically, the scattering factor is a concavo-convex shape or the like formed on the above boundary surface. That is, in this case, the scattering factor, which is formed in advance, corresponds to an interference fringe pattern stored as a hologram.
In order to retrieve a hologram stored in the medium shown in FIG. 77, a reference beam 5 is input from an end face of a core layer 1, so that light scattered by the scattering factor 32 (formed on the boundary surface of the core layer 1) is detected as a retrieved beam 7.
However, in the media shown in FIGS. 74 and 76, the above-explained evanescent light generally has a low light intensity; thus, data may not be sufficiently stored in the storage layer. In addition, the core layer 1 and the storage layer 4 are close to each other so that the reference beam emitted onto the core layer is also easily incident on the storage layer 4. As a result, the holographic storage and retrieval operation cannot be performed under preferable conditions. Furthermore, even if the retrieved beam 5 is incident only on the core layer 1, an evanescent light component included in the reference beam is absorbed into the storage layer 4. Therefore, the intensity of the reference beam (i.e., the evanescent light) attenuates while the input beam proceeds from the input face, thereby producing insufficient data storage in the storage layer 4.
Additionally, in the medium shown in FIG. 76, the storage layer 4 should be provided for each core layer 1; thus, the entire structure and manufacturing processes are complicated, thereby increasing the manufacturing cost.
In addition, the medium shown in FIG. 77 is a retrieval-only medium; thus, additional data storage or revision of stored data is impossible.
An another example of the optical waveguide is shown in FIG. 78, which is used as a grating optical coupler (refer to T. Kose et al., “Hikarikougaku Handobukku (Optical Engineering Handbook)”, Asakurashoten (Asakura Publishing Company), pp. 226–227, 1986). The optical coupler is used for coupling a light beam in the optical waveguide and a light beam transmitted through free space.
In FIG. 78, a core layer 1 is formed on a substrate 1010, and a diffraction grating layer 102 is further formed on the core layer 1. When an incident beam 103 (which may be called a reference beam) is input from an end face of the core layer 1, a transmitted beam 104 transmitted through the core layer 1 is diffracted by the diffraction grating layer 102, so that a diffracted beam 105 is output to free space at an output angle dependent on the diffraction grating layer 102. On the other hand, when a beam is input from free space into the core layer 1 at an incident angle dependent on the diffraction grating layer 102, a transmitted beam 104 can be transmitted through the core layer 1 (i.e., in the waveguide).
As shown in FIG. 78, when a transmitted beam 104, input from the left end of the core layer 1 of the optical waveguide, is transmitted in the core layer 1 from the left to the right side, the intensity of the transmitted beam 104 in the core layer 1 gradually attenuates due to absorption in the core layer 1 and diffraction by the diffraction grating layer 102. The intensity of the diffracted beam 105 is calculated by multiplication between intensity of the transmitted beam 104 and diffraction efficiency.
In such conventional optical waveguides, the diffraction efficiency of the diffraction grating layer 102 is uniform and has no specific distribution through the layer. Therefore, the diffracted beam 105 has a light intensity profile which is in proportion to the light intensity profile of the transmitted beam 104. That is, in the light intensity profile of the diffracted beam 105 (see FIG. 79), the left end has the largest value, and it gradually decreases to the right end.
Generally, in the above-explained conventional structure of the optical waveguide, a light beam transmitted through free space preferably has a light intensity profile which is as uniform as possible. However, in the conventional optical waveguide, a light beam in free space, that is, the diffracted beam 105 diffracted by the diffraction grating layer 102 has a light intensity profile which is not uniform through the layer; thus, it is difficult to handle this kind of optical waveguide as an optical component.